FIG. 1 schematically shows a cross-section of an MRI imaging magnet housed within a cryostat. As is well known in the art, such arrangements typically comprise a set of superconducting coils 10 mounted on a former (not shown), suspended in a cryogen vessel 12 which is partially filled with liquid cryogen 14. The liquid cryogen is selected such that its boiling point is below the superconducting transition temperature of the wire used in the coils 10. An outer vacuum container OVC 16 surrounds the cryogen vessel. The space 18 between the inner surface of the OVC and the outer surface of the cryogen vessel is evacuated, to reduce heat influx to the cryogen vessel by convection. One or more thermal radiation shields 19 may be provided in the evacuated space, to reduce thermal influx to the cryogen vessel by radiation. A solid thermal insulating layer such as aluminium coated polyester sheets 19a may also be provided within the evacuated space, to further reduce thermal influx. Careful design of support and suspension members 20 reduces heat influx to the cryogen vessel by conduction.
The coils 10 are provided with electrical current by current leads 22 leading into the cryogen vessel through an access turret 24. The access turret typically also provides a venting path 25 for cryogen gas to escape. It is necessary to allow cryogen gas to escape for several reasons, depending on the state of operation of the magnet which comprises coils 10. The present invention relates to the equipment provided to allow venting of cryogen gas. Some examples of situations which require the venting of cryogen gas are as follows. During operation, the cryogen vessel 12 must remain sealed against air ingress, yet the gas pressure within the cryogen vessel must be accurately controlled to maintain the correct thermal environment for the superconducting coils.
Control of cryogen gas venting during all normal operating conditions (cryogen fill, ramp, and operation at field) may be achieved using a direct-acting mechanical valve. Achieving the required control precision with such mechanical valves has proved difficult and expensive. Consequences of this poor control include less-than-ideal coil temperatures during ramping, with consequently increased risk of quench, and increased cryogen losses.
The mechanical vent valve depends on the balance of gas pressures and spring forces to modulate the opening of the valve's plate. The operating forces in a valve of this type are small, and consequently performance is sensitive to small changes in spring force, friction, operating temperature, and a range of manufacturing tolerances. Expensive calibration and conditioning techniques are used to reduce these effects, but despite this, pressure control performance is only just adequate for the application and reliability is poor.
Examples of such valves include a simple spring-loaded valve which opens when the pressure inside the cryogen vessel exceeds the pressure on the other side of the valve by an amount sufficient to overcome the bias force provided by the spring. The valve is opened by an excess of pressure on the cryogen-vessel side, and is closed by a spring acting against the pressure of the cryogen vessel. In the case of over-pressure within the cryogen vessel, the pressure acts on the valve against the spring to vent cryogen from the cryogen vessel. Should the pressure within the cryogen vessel fall below the pressure on the other side of the valve, typically atmospheric pressure, then that pressure will act on the valve in support of the spring to hold the valve firmly shut and prevent unwanted ingress of air to the cryogen vessel. A passive over-pressure and under-pressure protection is accordingly provided.
The internal cryogen vessel pressure required to open such valves may vary according to external influences. For example, the absolute cryogen vessel pressure required to open the simple spring loaded valve will vary with atmospheric pressure acting on the “downstream” side of the valve. While one may have designed and intended such mechanical valves to operate to maintain a certain absolute pressure within the cryogen vessel, variations in the absolute pressure required to open the valve will occur. This effect may be reduced by providing a selection of slightly different versions of the valves, and a precise valve type may be chosen from a range of such valves according to the expected atmospheric pressure and ambient temperature to be experienced by the cryogen vessel in operation. This is of course inconvenient, requiring multiple valve types and appropriate arrangements to select the correct valve for installation.
In summary, despite significant development efforts in the past, existing direct-acting vent valves (wherein the valve plate is directly operated by the pressure within a vessel or a spring, or similar) do not provide accurate or optimised control of cryogen vessel pressure for superconducting magnets for MR imaging, and similar apparatus. Due to demanding calibration requirements, such valves are also expensive to manufacture and unreliable in operation.
In known MRI imaging systems and the like, and as shown in FIG. 1, it is customary to provide a magnet supervisory system 30 which receives data from numerous sensors 32, 34; which is in control of electrical current flow in the magnet, and which controls the operation of the magnet system for optimal performance at all times: during ramp-up, in steady state operation; during imaging and during ramp-down.
It has been found difficult to provide a mechanical valve system with effective and reliable on/off operation. Even a small amount of contamination on the valve element or valve seat may cause the valve to leak in the closed position. On the other hand, contamination may prevent the valve from opening completely. In either case, the valve may not maintain the required pressure within the vessel or permit the required gas flow rate from the vessel.
Accordingly, it has been proposed to control venting by a valve 40 fitted with an actuation device, which is controlled using an intelligent controller such as magnet supervisory system 30, which has access to data defining the magnet operational status and existing conditions within the cryogen vessel. In the example illustrated in FIG. 1, sensors 32 and 34 provide data to the magnet supervisory unit 30 indicating the pressure within the cryogen vessel and the flow rate of gas venting from the cryogen vessel. With such an arrangement it is possible to optimise the operating pressure in the cryogen vessel and the vent gas flow rate from the cryogen vessel to suit the magnet operating conditions and/or existing conditions within the cryogen vessel, with consequent advantages including reduced quench risk and reduced losses of cryogen.
The controlled valve 40 may be operated whereby pressure within the cryogen vessel may be reliably maintained within a desired range of values, and/or a venting of gas may be operated when an internal absolute or gauge pressure reaches a certain value. An accurate and predictable control of vent gas flow rate from the cryogen vessel may be maintained as required.
To avoid the risk of air/ice contamination of the cryogen vessel, the pressure within the cryogen vessel 12 is normally maintained above atmospheric by magnet supervisory control system 30; for example by controlling an associated cryogenic refrigerator in accordance with pressure measurements provided by an absolute or gauge pressure transducer 32. However; during ramping, further pressure rise within the cryogen vessel must be strictly limited in order to maintain acceptable magnet temperatures by allowing increased boil-off of the liquid cryogen. As a result of these conflicting requirements, very precise control and measurement of cryogen vessel pressure is required to be provided by the vent valve 40 and the pressure control system, typically included within controller 30.
The controlled valve 40 may have a simple cyclic on/off function. Accurate pressure control is achieved by varying a duty cycle of the on/off state of the valve, eliminating the need for precise calibration of the valve. In such arrangements, the exact flow capacity of the valve is not particularly important, as the pressure measurement and duty cycle adjustment will compensate for minor variations. For example, a very simple control method may operate along the lines of:
(1) Set required absolute pressure within cryogen vessel=x;
(2) detect actual pressure p within cryogen vessel from sensor 32 conventionally provided;
(3) If p>x, increase “open” proportion of valve operation duty cycle; and
(4) If p<x, reduce “open” proportion of valve operation duty cycle.
Where control of gas egress flow rate is required, rather than gas pressure, the control method may resemble:
(1) Set required gas egress flow rate from the cryogen vessel=R;
(2) detect actual gas egress flow rate r from the cryogen vessel from a sensor 34 conventionally provided;
(3) If r<R, increase “open” proportion of valve operation duty cycle; and
(4) If r>R, reduce “open” proportion of valve operation duty cycle.
Suitable control signals and suitable arrangements for modifying the control signals to provide the required operation may be simply derived by those skilled in the art.
Various control strategies are possible for the valve and may be defined in the software of the control unit. An advantage of such arrangements is that imperfections in the valve hardware may be compensated for in the software of the magnet supervisory control system 30. The control unit conventionally receives data from various sensors fitted to the magnet and elsewhere within the cryogen vessel, indicating such parameters as absolute cryogen vessel pressure and liquid cryogen level. As will be apparent to those skilled in the art, one may employ data provided by such sensors to operate the actively controlled valve of the present invention to control the absolute pressure inside the cryogen vessel within a desired range of values as required for any of a number of operational situations and requirements. The required valve control may be based on measurements of cryogen vessel pressure. Such pressure measurements can be either absolute or gauge (that is, relative to atmospheric pressure), and the type of transducer used can be selected for use in different operating conditions if desired. By providing an atmospheric pressure sensor, the gauge pressure of the cryogen vessel may be controlled. Alternatively, the actively controlled valve may be operated to maintain a gas egress flow rate within a desired range of values.
Control signals for valves operated by stepper motors, generated by the magnet supervisory control system 30 may easily be derived by those skilled in the art.
For current MRI imaging magnet systems, it has been found that a cyclic valve on/off function at an operating frequency of below 1 Hz is quite sufficient, given the size of the system. Greater frequencies of valve operation may be found necessary, particularly for much smaller cryogen vessels 12.
Typically, the valve element is directly operated by a solenoid. FIG. 2 shows a conventional arrangement of a solenoid valve 42 provided to control cryogen vessel pressure, or rate of cryogen gas egress flow. In FIG. 2, the solenoid valve 42 comprises an actuating coil 44 and an armature 46 carrying, or serving as, a valve element. According to the magnitude and/or direction of electrical current supplied through leads 48 to the actuating coil 44, the armature 46 will move to bring the valve element into contact with, or away from, a valve seat 50. A weak spring 52 urges the valve element towards the valve seat to close the valve when the solenoid is not activated. While the spring is schematically illustrated as a coil spring, any known resilient member may be used as appropriate, such as a leaf spring, a conical spring, a helical spring, or resilient members such as a deformable rubber member. In FIG. 2, the valve is illustrated in a mid-way position, between a fully-open position and a fully-closed position. This is for illustration purposes only, and does not represent a stable position for a solenoid valve.
FIG. 3A shows the valve 42 in a fully open position, typical of the position of a solenoid valve in its “actuated” position—with an actuating current flowing in actuating coil 44.
FIG. 3B shows the valve 42 in a fully closed position, typical of the position of a solenoid valve in its “rest” position—with no current flowing in actuating coil 44. The valve element is urged into contact with the valve seat 50 by spring 52; possibly also by gravity.
In a system such as illustrated in FIG. 1, control system 30 controls the valve so as to maintain a pressure within the cryogen vessel 12 within a desired range of values, or a gas egress flow rate within a desired range of values by the controller changing the on-off time ratio (duty cycle) of cyclically opening and closing the valve 42 by appropriately controlling the current in actuating coil 44. By using a suitably dimensioned valve and operating frequency, the variation in pressure can be maintained within close limits.
In the case of an unexpected over-pressure within the cryogen vessel 12, while the valve is closed (FIG. 3B), the pressure will act on the “upstream” surfaces of the armature 46 and the valve element, to urge the valve tightly into its closed position, with the valve element on the valve seat. This will prevent or impede venting of boiled off cryogen gas from the cryogen vessel, possibly leading to an unwanted build-up of pressure within the cryogen vessel. Such an increase in pressure may lead to an increase in the temperature of the gas, and so raise the temperature of the coils, risking a quench.
Conversely, in the case of an under-pressure within the cryogen vessel 12, while the valve is closed (FIG. 3B), bringing the pressure within the cryogen vessel below the pressure external to the cryogen vessel, the pressure external to the cryogen vessel will act on a surface of the armature 46 and the valve element, to urge the valve open. This may allow the unwanted ingress of air or other contaminants into the cryogen vessel 12 and/or the venting path 25. Such leakage is to be avoided since the cryogen vessel is typically at a temperature well below the freezing point of the major components of air. The ingress of air or other contaminants may cause deposits of frozen contaminants such as nitrogen and water to form within the cryogen vessel 12 and/or the venting path 25. This may cause a blockage which may prevent or impede later venting of boiled off cryogen gas from the cryogen vessel, possibly leading to an unwanted build-up of pressure and an associated increase in temperature within the cryogen vessel at a later time. This may cause a quench, with cryogen unable to vent satisfactorily due to the blockage, causing dangerously high pressure within the cryogen vessel. Solid contamination of the valve may also make pressure control more difficult, while contamination of the magnet structure may cause quench events.
For these reasons, use of a controlled solenoid valve such as discussed above and illustrated in FIGS. 2-3B has been avoided in the prior art, and the use of direct-actuated mechanical valves has remained current, &spite their disadvantages discussed above.